DESCRIPTION: The proposed studies seek to establish the functional circuitry underlying the upper threshold units found in the mammalian inferior colliculu Most auditory brainstem units increase or maintain their response as stimulus intensities are raised above threshold levels. Upper threshold units are characterized by a response decrease or elimination as stimulus intensity is raised 30 to 40 dB above best frequency threshold. Upper threshold units are common in the inferior colliculus and the dorsal cochlear nucleus, but less common elsewhere in the auditory brainstem. The first aim will test whether t dorsal cochlear nucleus is the source of the upper threshold response properti seen in the inferior colliculus. Besides having similar unit properties, the dorsal cochlear nucleus projects to the contralateral colliculus via its main output tract: the dorsal acoustic striae. After isolating and characterizing units in the inferior colliculus, lidocaine will be applied to dorsal acoustic striae to reversibly block the input from the dorsal cochlear nucleus. Reversible alteration or loss of upper threshold properties in collicular unit following dorsal acoustic striae blockage would implicate the dorsal cochlear nucleus as a source of this response pattern. To further implicate dorsal cochlear nucleus in the activity of recorded collicular units, some experiment will include stimulation of somatosensory inputs to the dorsal cochlear nucleu while monitoring collicular response patterns. The second aim will test the most obvious alternate hypothesis: that the upper threshold inhibition pattern is built up within the inferior colliculus on the basis of local interactions excitatory and inhibitory inputs. Under this aim, recorded unit properties within the colliculus under normal conditions will be compared with properties of the same units after local application of pharmacological blockers (bicuculline or strychnine) of the inhibitory neurotransmitters GABA or glycin Studies will be carried out in a decerebrate preparation to limit possible anesthetic effects that could alter dorsal cochlear nucleus outputs.